How do I start a digital business from an existing product/service I provide?

Great question, I have launched a few digital products and have helped others do the same.

The steps can be as simple as:
Setup a PayPal link and send it out to people or can involve complex applications. Without knowing the exact nature of the product or service in mind here are some general steps.
1. If you don't own your business domain and have a website up, that would be the first step.
2. There are several options for payment. If you have a digital product that will be delivered Gumroad has an excellent platform for selling digital items such as ebooks, videos, MP3. There is also selz.com which allows products as well as services.
3. Drive traffic to your page via social media or paid advertising.
4. As this is an existing product, you may have some ideas of where the market hangs out in terms of blogs, forums, chat rooms, etc.. Remember, if going into a group you want to offer value before trying to sell or it will likely backfire.

I hope that helps with a bit of a framework. If you have further questions let me know or you can schedule a Clarity call with me to help plan this out in further detail.

In addition to the response above, I would suggest that you may also want to look at whether the existing product lends itself to multiple forms of digital products. For example, could a published book in hard copy become an ebook, as well as a set of short videos, maybe an audio recording or podcast, or could subsets of the written piece become smaller, bite-sized information products in which the content is somewhat re-purposed for a specific niche or audience?

Digital products take many forms and each individual buyer prefers a different method for consuming your content. For example, I almost exclusively will listen to audiobooks vs. reading something because I have so many time constraints. If you can offer a suite of related products on platforms that are diverse, you have the best chance of reaching a much larger audience.

What current market, industry , niche or ecommerce are you in? Each requires a series of marketing tactics you can use to attract appropriate customers to your landing page and convert into buying customers.